Surface mining is the primary method for recovering a variety of valuable minerals including: phosphate rock, feldspar, potash, rutile, and titanium from ore deposits. These minerals generally occur in nature in admixture with argillaceous material, silica, and extraneous gangue and are usually covered with as much as twenty to thirty feet of earth, sand, rocks, and vegetation which are referred to as "overburden" by the mining industry.
In order to recover the valuable minerals from the deposit, it has been a conventional practice of the industry to remove the overburden, generally with the aid of draglines, steam shovels or the like, and then using the same or similar equipment to dig the ore from the deposit, casting it into an earthen sump where it is slurried with a stream of high pressure water. Typically, the slurry has been withdrawn from the sump and pumped under high pressure to an ore dressing plant where the slurried matrix is crushed, deslimed, screened and subjected to a flotation process for separation of the valuable minerals from the undesirable constituents of the ore. The undesirable constituents are usually referred to as "slimes" and "tailings" and both have created major disposal problems for the industry since the development of the flotation process in the 1930's. The slimes are aqueous suspensions of ultrafine solid wastes, primarily clay particles, and the tailings are essentially water-insoluble granular particles of sand or quartz.
In the early days, slimes were simply deposited in excavated mining pits or in damned reservoirs where they were permitted to settle and left as lakes in which the bottoms were covered with as much as 20 to 30 feet of unsolidified slimes that never compacted into a solid lake bottom. As such, these lakes were hazardous to anyone who used them. Therefore, in an effort to eliminate such hazardous conditions, water was drained from several of these lakes only to find that a crust formed over the drained slimes, and the slimes beneath the crust never solidify sufficiently to form a stable land mass. For unsuspecting travelers or animals that venture into these drained and crusted areas, this practice created even more hazardous conditions than the lakes with their jelly-like bottoms.
Another problem that has plagued the surface mining industry since the 1930's is the disposal of tailings. Although the disposal of tailings from beneficiation of ore does not produce the hazardous conditions associated with slimes disposal, it has been found that tailings from beneficiation processes are generally devoid of plant nutrients, and use thereof as landfill generally results in production of unstable soil and creation of areas that do not support normal or rapid revegetation.
In an attempt to overcome the above problem and reclaim some of the mined-out properties, several mining operations have employed a reclamation system which is now referred to as lake-land reclamation. In this method of reclamation, portions of the mined out depressions are backfilled with overburden to bring the land back to approximate grade level and the remainder of the depression is converted to a lake. Neither the slimes nor the tailings are utilized in this reclamation system. Rather, in this system the slimes are impounded in reservoirs frequently above ground level and the tailings are simply stackfilled. Water is drained from the settled slimes and then slowly eliminated by ditching around the perimeter of the impounded settled slimes. This system requires continuous attention to proper ditching and immediate repair to the earthen dams which are usually employed to retain the slimes.
As such, there has been a continuing effort over the years to find a satisfactory method for disposing of the waste products obtained from flotation processes and for restoring mined-out properties to a usable condition utilizing the waste products therefrom. However, it was only about a decade ago that significant improvements in the technique of land reclamation began to appear. These improvements constitute the initial entry of the industry into what is now being referred to as "sand-clay" reclamation. They are described in U.S. Pat. Nos. 3,718,003; 3,761,239; 3,763,041; 3,940,071 and 4,036,752, and have been used successfully to restore substantial areas of mined out property to usable condition.
Although these patented processes have met with some success, unfortunately, they are not entirely satisfactory. They have not provided a means for utilizing all of the clay or slimes separated from the ore in the flotation process, nor have they proved to be entirely satisfactory in achieving a recombination of the clay with the sand in a sand-clay ratio approximating the 2:1 ratio found in most Florida phosphate deposits. In practice we have found that the patented processes have generally achieved a sand-clay recombination of only about 5 to 6 parts sand to 1 part clay.
Thus, it is an object of the present invention to provide a novel process for the restoration of mining excavations using sand and clay waste products from an ore flotation process where in the ratio of sand to clay is between about 3:1 to 4:1.
As the demographic pressures of expanding populations continue to increase in the vicinity of surface mining areas, still more effective methods of waste disposal and land reclamation are required to accommodate the needs of the expanding population and to comply with recent county and state environmental regulations. Long-range stability of reclaimed land thus becomes increasingly important since long-range stability is required to restore the uninhabitable, useless land of an excavated mine site to a firmness and fertility which can be utilized for recreation, farming, reforestation and, ultimately, to building sites for housing development. It is to this end that the process of the present invention is directed.